1. Field of the Invention
This invention pertains generally to dehydration devices and methods and more particularly to a desiccation method and apparatus with multiple dehydration zones utilizing ultrasound, heated circulating air and a substrate matrix.
2. Description of the Background Art
The preservation of food and other organic and inorganic material by the evaporation of water from the material is well known in the art. Dehydration allows food to be kept for longer periods of time without refrigeration. The size and weight of the food is reduced by dehydration and the cost of transportation and storage of the food is therefore minimized.
Early methods of dehydration consisted of placing whole or diced food articles on trays and setting the trays in the sun for several days to allow the food to dry. This method proved to be undesirable on a commercial level because of the accumulation of dust, molds and other air-borne particles on the food as well as the discoloration of the food that often occurs upon exposure of food to ultra-violet light. Furthermore, microbial levels in sun-dehydrated foods were often unpredictable and unacceptable with these early methods.
Mechanical kiln type dehydrating devices that isolated the food from sunlight and outside air were then developed. These devices passed heated air through perforated trays until the water content of the food particles was reduced to a desired level. However, these methods did not appreciably change the presence of microbiological contaminants in many dehydrated foods, particularly those that were dried at relatively low temperatures for comparatively long periods of time.
Although an improvement over sun dehydration, the kiln type dehydration devices still produce discolored foods in many instances due in part to the length of time required to dry the foods. In order to preserve the natural color and texture, many dehydrated fruits were treated with sulfur dioxide, sulfites or other chemical preservatives. For many people, the taste of the preservatives made the treated foods undesirable. For others, the preservatives posed a health risk leading to legislation limiting the amount and types of preservatives that could be present in various dehydrated foods.
Later methods sought to eliminate enzyme activity and reduce the levels of bacteria and the like by blanching the food with steam or hot water and then drying the food at high temperature. Unfortunately, blanching may alter the flavor and texture of some foods and may make other foods difficult to dehydrate because the food absorbs water during the blanching process. Likewise, some foods are sensitive to exposure to heat. High drying temperatures may also adversely affect the color and flavor of dehydrated foods. Furthermore, blanching methods are not always effective in consistently reducing the microbial levels to acceptable levels.
Recently developed methods of dehydration include treating the food with an osmotic agent and then dehydrating the food with heated air. Still other methods use heated vegetable oil and treatment in reduced pressure environments. These methods are unsatisfactory due to the residues left by the treating agents as well as the expense of production.
Substantially microbe free dehydrated foods have been produced by “freeze-drying” methods known in the art. Fruit and vegetable products are typically frozen and the water removed by sublimation in a low-pressure environment with these methods. The cost of high capacity refrigeration systems and low-pressure systems, as well as the cost of energy and maintenance, makes the resulting food product expensive to manufacture using these methods.
Some seasonal vegetables, such as onions and bell peppers have a limited market life. For example, onions that are beyond certain size limits are often tilled under in the field or composted because the onions cannot be brought to market during the season. As much as twenty-five percent of the yearly onion crop may be wasted in this manner.
Only a small percentage of onions are currently dehydrated because of the difficulty experienced dehydrating onions using current methods. Presently, yellow onions may be frozen to preserve the onion until the onions can be processed. In addition, diced pieces of onion do not dry well because the pieces tend to stick together due to the sugar content of the onion thereby creating pockets of moisture. Bacteria are found in such moisture pockets requiring the destruction of the onion pieces resulting in additional waste and expense.
Materials other than food, such as medicinal herbs, may be prepared using dehydration to provide material for encapsulation or the like. Dehydration may also be used in the processing of sludge or other organic matter as well as inorganic matter. Accordingly, the principal challenge to current desiccation methods is to generate a dehydrated product with natural colors, textures and flavors that is free from microbiological contamination and noxious residues. Thus, there is a need for an effective and cost efficient desiccating apparatus and method that can maintain the natural color, flavor and texture of the food while keeping the microbial level within acceptable limits without using additives or preservatives or costly desiccation machinery and methods. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in existing equipment and methods.